The present invention relates to an activated sludge wastewater treatment process. More particularly, the invention relates to multivalent metal ion management for low-yield wastewater treatment processes.
Organic water pollutants can often be removed by biological wastewater treatment, that is, by cultivating bacteria to convert most of the organic pollutants in the wastewater to carbon dioxide, water and bacterial cell mass (“biomass”). This biological process is similar to that used by the human body to convert food into heat, muscle mass and motion.
A conventional activated sludge wastewater treatment process is shown in FIG. 1. The process generally involves cultivating within an aeration reactor a “mixed liquor” of bacterial cells suspended in wastewater. The bacterial cells are only slightly denser than water, and so are easily maintained in suspension (“suspended growth”). Solid-liquid separators, such as large quiescent clarifiers or membrane-based systems, are typically used to separate the cultivated mixed liquor into a liquid suspension of biomass (“activated sludge”) and a clear effluent. The clear effluent may be removed from the waste stream and discharged into a local waterway. At least a portion of the activated sludge may be recycled to the aeration reactor as return activated sludge (RAS). The RAS helps maintain a sufficient concentration of bacterial cells in the aeration reactor for effective cleaning of the incoming wastewater. Since conversion of the readily degradable solids creates additional biomass within the wastewater treatment system, a portion of the activated sludge is typically removed from the plant as waste activated sludge (WAS) to maintain the biomass within an acceptable performance range.
Current wastewater treatment processes exhibit two important drawbacks. First, many wastewater treatment processes generate large amounts of WAS that must be sent off-site for disposal. Handling and disposal of WAS is typically the largest single cost component in the operation of a wastewater treatment plant. Second, most wastewater treatment processes cannot effectively respond to diurnal, seasonal, or long-term variations in the composition of wastewater. A treatment process that may be effective in treating wastewater during one time of the year may not be as effective at treating wastewater during another time of the year. Therefore, a need exists within the industry for wastewater treatment processes that both lower WAS and adjust effectively to fluctuations in wastewater composition.